The present invention relates to an line pressure control device, and more particularly to a line oil pressure control device for a continuous transmission in an automotive vehicle, wherein a line oil pressure is obtained from an output oil pressure of an oil pressure pump driven by an internal combustion engine, and the line oil pressure is controlled to be a pressure corresponding to a transmission torque by controlling the output oil pressure.
A prior art configuration of a continuous transmission, also known as a continuously variable transmission or CVT, in an automotive vehicle is disclosed, for example, in Japanese laid open Patent No.4-272569. As is shown in this art, the configuration is frequently used, wherein a belt is tightened over a pair of pulleys, and a rotation ratio of a driving side and a driven side is controlled by mutually changing a gap width of the pair of pulleys.
A mechanism for changing the gap width of the pulleys will now be described. First, a line oil pressure is procured from an oil pressure of an oil pressure pump driven by an internal combustion engine laden in a vehicle. Second, the line oil pressure is changed by controlling an output oil pressure of the oil pressure. Finally, a movable conical disk making up the pulleys is displaced in the axial direction towards a fixed conical disk according to a change of the line oil pressure.
In a control of the line oil pressure of the continuous transmission in the vehicle of this kind, it is required to make an effective transfer of a driving force between the belt and the vehicle. Therefore, it is absolutely necessary for obtaining an oil pressure for the belt, wherein a durability of the belt is not damaged by strictly lessening a slip of the belt between the driving and the driven sides. Furthermore, it is indispensable to suppress a power loss of the oil pressure pump by preventing the line oil pressure from being raised above a required level. To overcome these difficulties, a method is commonly used wherein an aimed line oil pressure according to the running conditions is set, and the output oil pressure of the oil pressure pump driven by the internal combustion engine is controlled to coincide with the aimed line oil pressure as is shown in a flow chart of FIG. 6.
In step 601 of FIG. 6, a base value of the aimed line oil pressure is computed from the input information to a controller concerning a torque of the internal combustion engine and a transmission ratio of the continuous transmission. Next in step 602, a compensating value for the base value of the aimed line oil pressure is computed based on the information from the various sensors detecting the running conditions. In step 603, a final aimed line oil pressure is computed by adding the compensating value with the base value of the aimed line oil pressure, and in step 604, all these steps are repeated periodically.
In the prior art as described above, in setting the aimed line oil pressure, it is required to set a rotating speed of the internal combustion engine as small as possible to improve a gasoline mileage, and simultaneously to raise the rotating speed of the engine for avoiding the slip of the belt due to a shortage of the output oil pressure from the oil pressure pump so that the line oil pressure corresponding to a transmission torque is obtained. Therefore, to satisfy these contradictory conditions, the rotating speed of the internal combustion engine is set so as to obtain a minimum aimed line oil pressure in a range wherein no slip of the belt occurs, and also, the setting of the internal combustion engine speed must be executed to cause no slip of the belt under variously and constantly changing running conditions of the automotive vehicle.
However, as shown in FIG. 7, the oil pressure pump is driven by the engine whose rotating speed is shown as a curve E, a possible output line oil pressure shown as a curve F becomes small in a low rotating zone, and a margin for the aimed line oil pressure shown as a curve G is small. Therefore, when the running conditions of the vehicle having a conventional control device of the line oil pressure is changing, the engine is unable to follow a change of the aimed line oil pressure by means of raising the output oil pressure of the oil pressure pump. Accordingly, a zone T takes place, wherein the belt wears by the slip, and a change of the running conditions due to the slip is inevitable. The zone T is defined between timing points t1 and t2. At the timing point t1 in the raising portion of the aimed line oil pressure, aimed line oil pressure is equal to the possible line oil pressure, and the timing point t2 in the raising portion of the possible line pressure, possible line oil pressure is equal to the aimed line pressure.
In view of the above, it is the object of the present invention to provide an oil pressure control device for a continuous transmission in an automotive vehicle, wherein a slip of a belt of the continuous transmission is capable of being avoided, and a life of the belt is lengthened.
According to this invention, a line pressure control device for a continuous transmission for an automotive vehicle, wherein a continuous transmission mechanism is actuated by an controlled oil pressure in an oil pressure line and outputs a transmitted speed from a rotating speed of an internal combustion engine, the line pressure control device comprises:
an oil pressure pump actuated by the internal combustion engine and generating an output oil pressure in the oil pressure line,
a pressure control means for generating the controlled oil pressure in the oil pressure line by controlling the output oil pressure of said oil pressure pump,
a throttle opening sensor for generating a throttle opening signal based on a opening of a throttle valve in the internal combustion engine,
an engine rotating speed sensor for generating an engine rotating speed signal based on a rotating speed of the internal combustion engine,
a transmission rotating speed sensor for generating an input rotating speed signal and an output rotating speed signal based on an input rotating speed and an output rotating speed of the continuous transmission mechanism respectively, and
a controller including a memory for memorizing the output oil pressure characteristics for a rotating speed of said oil pressure pump,
wherein said controller computes an input torque to the continuous transmission mechanism, a required line oil pressure for transmitting said input torque from said throttle opening signal, said input rotating speed signal and said output rotating speed signal, and a possible output line oil pressure from said output oil pressure characteristics and said engine rotating speed signal, and
said controller controls the speed and output torque of the internal combustion engine so that said required line oil pressure becomes no more than said possible output line oil pressure by comparing said required line oil pressure with said possible output line oil pressure.
Furthermore, the line pressure control device for the continuous transmission mechanism in the automotive vehicle,
wherein if said possible output line oil pressure is judged smaller than said required line oil pressure, said controller controls in a way to raise said speed of the internal combustion engine, and lower a transmission ratio of the continuous transmission mechanism, thereby decreasing said output torque of the internal combustion engine.
The line pressure control device for the continuous transmission mechanism in the automotive vehicle in accordance with this invention is adapted to avoid a slip of a belt of the continuous transmission mechanism and to lengthen a life of the belt, by increasing a rotating speed and an output torque of the internal combustion engine according to the need, while the rotating speed and output torque of the internal combustion engine are possibly maintained to lower value, and then to obtain more smooth transmission even if a running condition is changed.